1. Field of the Invention
The present invention relates to an organic electroluminescent device and a display unit. More particularly, the present invention relates to an organic electroluminescent device having a long light-emitting life and also to a display unit provided with such devices.
2. Description of the Related Art
Display units of flat panel type have prevailed recently because of their light weight and high efficiency. Noteworthy among them is one which is provided with organic electroluminescent devices (so-called organic EL devices).
The organic electroluminescent device is formed on a transparent substrate such as a glass plate. The device proper includes an anode and a cathode and an organic layer held between these electrodes. The anode adjacent to the substrate is formed from transparent ITO (indium tin oxide). The organic layer is composed of a hole injection layer, a hole transport layer, an electron transport layer, and a light-emitting layer, which are placed one over another. The cathode is formed on the top. This structure may be modified such that an additional electron transport layer is placed on the light-emitting layer. The organic electroluminescent device constructed as mentioned above works in such a way that the light-emitting layer causes recombination to occur between electrons injected from the cathode and holes injected from the anode and recombination emanates light through the anode and substrate.
There is another organic electroluminescent device differing in structure from the foregoing one. It is composed of a substrate, a cathode, an organic layer, and an anode, which are sequentially arranged upward. The top electrode (anode or cathode) may be formed from a transparent material so that light emanates from the top electrode opposite to the substrate. The device of top-emission type is desirable for the display unit of active matrix type, which is composed of a substrate and thin-film transistors (TFT) arranged thereon, because of its large open area ratio.
For the organic electroluminescent display unit to be of practical use, the organic electroluminescent device should have a large open area for efficient light emission as well as a reliable long life.
There are several factors detrimental to life. Noteworthy among them is deterioration of the electron transport layer which is usually made of Alq3 (8-hydroxyquinoline aluminum). In Science, 1999, vol. 283, pp. 1900-1902, Z. D. Popovic et al. reported that the organic electroluminescent device is deteriorated by hole injection into Alq3 and radicals resulting therefrom, and also proposed a model of deterioration.
Based on the model of deterioration, Z. D. Popovic et al. attempted to suppress hole injection into Alq3 by doping the hole transport layer with a low-molecular weight compound (such as rubrene). This attempt succeeded in a great improvement of emitting life. The result proved the model assuming that the doping layer traps excess holes, thereby preventing Alq3 from forming radicals. See Thin Solid Films, 2000, vol. 363, pp. 6-8.
Moreover, it is reported in Applied Physics Letters, 2002, vol. 80, pp. 2180-2182 that the doped rubrene produces no effect at the interface between the laminated layers (for example, between the metal layer and the organic layer or between the organic layer and the organic layer) but the layer doped with rubrene changes in its bulk properties and traps holes, thereby extending life.
The technology involving rubrene is not limited to the trapping of charges as mentioned above. It is also directed to the organic electroluminescent device that emits white light because rubrene, in the form of simple substance, emits yellow light and hence a combination of rubrene layer and light-emitting layer that emits blue light would produce white light. See JP-A-2002-93583; JP-A-2004-47469; JP-A-2004-134396; JP-T-2007-503092; Science, 1999, vol. 283, pp. 1900-1902; Thin Solid Films, 2000, vol. 363, pp. 6-8; Applied Physics Letters, 2002, vol. 80, pp. 2180-2182; Applied Physics Letters, 2003, vol. 83, p. 5359-5361; Applied Physics Letters, 2006, vol. 89, p. 133509; and Applied Physics Letters, 2006, vol. 89, p. 243521. The organic electroluminescent device with rubrene will produce white light whose purity depends on the ratio between light from the rubrene-doped layer and light from its adjacent light-emitting layer that emits blue light because both layers emit light simultaneously.